1. Field of the Invention
The present invention relates to a power line communication apparatus, a power line communication method, and a communication apparatus that have a function to convert multicast communication into unicast communication and that are capable of performing power line communication (hereinafter referred to as PLC) at high quality.
2. Description of Related Art
PLC that performs communication over indoor power lines has recently drawn attention. A currently proposed system even performs communication on power lines in a frequency range of 2 MHz to 30 MHz at a maximum transmission rate of over 100 Mbps in an OFDM modulation scheme, and thus a focus is on access to the Internet and the like from within a LAN and reception of video and audio stream distribution.
In PLC, however, the power lines are not originally designed for data communication. In addition, a number of branch wirings are installed, and a number of electric appliances are connected. Thus, the communication environment changes dynamically. Further, noise, reflection, attenuation, and the like cause data loss, thus resulting in traffic slowdown, transfer delay, and the like, and, in stream distribution, video and audio distortion and communication quality degradation. Proposed to address the problems is a PLC modem that employs impedance matching between the modem and a transmission line so as to maintain communication quality (e.g., Japanese Patent Laid-open Publication 2003-244038).
Further proposed is a PLC modem capable of maintaining communication quality without losing a frequency band (e.g., Japanese Patent Laid-open Publication 2002-314466). The PLC modem on a video transmitter disclosed in the publication simultaneously transmits a plurality of signals in a frequency division multiplexing system. Using two types of packets, an original packet and a duplicate packet, the PLC modem distributes risk of signal degradation and thus maintains the communication quality.
Even with such PLC modem, however, it is inevitable that communication environments where PLC modems are installed are statically and dynamically different by location on the PLC network. It is thus impossible that one PLC modem performs quality communication at an equal transmission rate to all of the plurality of PLC modems. Therefore, simultaneous multiple transmission, such as broadcast and multicast, is generally performed at a low rate (about 2 Mbps), which allows the most stable communication.
Multicast is generally used for TV broadcast and other video and audio stream distribution over an IP network, such as the Internet and the like. In transmission of packets for TV broadcast and the like to an IP device over the PLC network, quality degradation (image distortion) has occurred in a high-bit-rate program (2 Mbps or more) due to data loss.
As described above, the communication quality deteriorates in PLC, since the power lines used are not originally designed for data communication; a number of branch wirings are installed; and a number of electric appliances are connected. Although the PLC modems disclosed in the above-described publications can improve the communication quality to a certain level, the PLC modems cannot ensure the communication quality.
That is, it is impossible that one PLC modem performs quality communication at the equal transmission rate to all of the plurality of PLC modems, since the PLC modems are provided in different communication environments due to the branch wirings and electric appliances connected to the power lines. Thus, the most stable communication at the low rate (about 2 Mbps) is employed for simultaneous multiple transmission, including broadcast and multicast. However, multicast is generally employed in TV broadcast and other stream distribution over the IP network, such as the Internet and the like, which are performed at the high bit rate.
Therefore, image and sound are inevitably distorted in the high-bit-rate program due to quality degradation caused by data loss, when the packets for TV broadcast and the like on the IP network are transmitted to the IP device via the PLC network.